Revert of Use in-object fields instead of private symbols for regexp slots. (patchset #4 id:60001 of https://codereview.chromium.org/1428203003/ )

Reason for revert:
browser_tests failure with --gtest_filter=ExternallyConnectableMessagingTest.EnablingAndDisabling

Original issue's description:
> Use in-object fields instead of private symbols for regexp slots.
>
> R=bmeurer@chromium.org
>
> Committed: https://crrev.com/5a1e42c039ac3379ebe1e7e34fb8163e1ec1493e
> Cr-Commit-Position: refs/heads/master@{#31791}
>
> Committed: https://crrev.com/bf5c9af92ac0a5b7f020ac968d3d42ed06aa6144
> Cr-Commit-Position: refs/heads/master@{#31805}

TBR=bmeurer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1432453005

Cr-Commit-Position: refs/heads/master@{#31817}